Alkali metal hydroxide solutions and chlorine are generally manufactured in mercury cells or diaphragm cells. Mercury cells have the advantage of producing concentrated alkali metal hydroxide solutions but give rise to problems associated with the disposal of mercury-containing effluents. On the other hand, diaphragm cells, in which the anodes and cathodes are separated by porous diaphragms which permit the passage of both positive and negative ions and of electrolyte, avoid the aforesaid effluent problem, but have the disadvantage that:
(1) relatively weak impure alkali metal hydroxide solutions are produced, which results in increased evaporation costs; and PA1 (2) there is a possibility of product gases, namely hydrogen and chlorine, becoming mixed. PA1 TFE=tetrafluoroethylene PA1 FEP=copolymer of tetrafluoroethylene and hexafluoropropylene PA1 PFBA=perfluorobutenoic acid PA1 IEC=ion exchange
Attempts have been made to overcome disadvantages of both mercury cells and diaphragm cells by the use of cells in which the anodes and cathodes are separated by cation-active permselective membranes; these are membranes which are selectively permeable so as to allow the passage of only positively charged ions and not the passage of bulk electrolyte. Cation-active permselective membranes which are suitable for this use in chlorine cells include, for example, those made of synthetic organic copolymeric material containing cation-exchange groups, for example sulfonate, carboxylate and phosphonate.
In particular, synthetic fluoropolymers which will withstand cell conditions for long periods of time are useful, for example, the perfluorosulfonic acid membranes manufactured and sold by E I DuPont de Nemours and Company under the trade mark "NAFION" and which are based upon hydrolyzed copolymers of perfluorinated hydrocarbons (for example polytetrafluoroethylene) and fluorosulfonated perfluorovinyl ethers.
The active sites in the molecular structure of the resins from which these membranes are made are provided by the fluorosulfonated perfluorovinyl ether component. These sites are present on side chains attached by an ether linkage to the skeletal structure of the resin. Such membranes are described for example in U.S. Pat. Nos. 2,636,851; 3,017,338; 3,496,077; 3,560,568; 2,967,807; 3,282,875 and UK Patent No. 1,184,321.
Generally these fluoropolymers are made by the polymerization or copolymerization of fluorocarbon monomers in an emulsion or suspension containing a radical polymerization catalyst. The resulting polymers are moulded into membranes by conventional moulding procedures such as melt fabrication.